Several things
Check the bentley on rear seat back, seat belt anchor and speaker removal. Any of these could rattle.
Note there are some sort of clips behind the seat back holding the panel in place. If they are loose, I supect the panel would rattle big time.
I need to remove mine for recovering this spring.

Originally Posted by
onewhippedpuppy
Is there anything back there that can be tightened, etc? I get an annoying rattle over big bumps or when slamming my doors.
Thanks,
1995 525i Auto, M50TU 2.5L, EAT chip, 1/95 build, USA, 205/65/15 tires, ASC+T, HID, lumbar, EC Mirror, BMW Alpine 5 radio with BMW-Pioneer CD Changer, abt 236k miles, Oxford Green/Parchment